This invention relates to side pincushion correction for a deflection circuit.
In a television receiver, the electron beams traveling inside the television receiver picture tube are deflected by the magnetic fields generated by sawtooth deflection currents flowing in horizontal and vertical deflection windings. The deflected electron beams scan a raster pattern on the picture tube phosphor screen. The raster pattern, without correction, may display various geometric distortions, such as side or east-west pincushion distortion and top and bottom or north-south pincushion distortion.
To correct side pincushion distortion, the peak-to-peak horizontal deflection trace current flowing in the horizontal deflection winding is modulated at a vertical rate in a parabolic manner. For large deflection angle picture tubes, such as 100.degree. or 110.degree. deflection angle picture tubes, such parabolic modulation is accomplished by a modulator circuit, such as a diode modulator, coupled to the horizontal deflection winding.
In a typical diode modulator circuit, the deflection trace capacitor is coupled to the deflection winding, and a modulator inductor is coupled to a modulator capacitor. The two capacitors are serially coupled to and charged from a B+ voltage supply through a flyback transformer primary winding. A deflection switch and modulator switch are provided in order to generate horizontal rate sawtooth currents in the deflection winding and in the modulator inductor. During horizontal retrace, the horizontal deflection winding and the modulator inductor resonate with retrace capacitors for providing flyback of the sawtooth currents.
A modulator control circuit coupled to the modulator capacitor varies the voltage across the capacitor at a vertical rate in a parabolic manner. As a result, the trace voltage across the trace capacitor will vary in a similar manner, thereby providing side pincushion raster correction.
With the flyback transformer primary winding coupled in the charging path of the trace and modulator capacitors, variations in loading by circuits magnetically coupled to the primary winding may undesirably modulate the trace and modulator capacitor voltages. The modulator control circuit should be designed to respond to these load variations in a manner which will substantially reduce the undesirable load current modulation.